(1) Field of the Invention
The present invention relates to a graphic display control system which performs multi-window display at a workstation or the like.
(2) Description of the Related Art
In a system in which a plurality of workstations are connected to a LAN or to other networks, a workstation acting as a display server sometimes displays the information sent from a plurality of workstations on the same screen. In such a case, information is generally displayed so that one window corresponds to each workstation from which information is sent, by means of a multi-window system.
FIG. 1 shows a schematic configuration of a conventional graphic display control system which performs multi-window display. Each of frame memories 2 and 3 has a memory capacity for one screen and retains display information for each window. A frame selection control memory 4 stores frame selection information, for each pixel or charaction section, as to which of frame memories 2 and 3 should be selected. A frame selection means 5 selects one of the data read out from the frame memories 2 and 3 on the basis of the frame selection information read out from the frame selection control memory 4, and sends it to the display unit 1 for display. Thus, the frame selection information is read out from the frame selection control memory 4 for each pixel or character section, and either of the data of frame memories 2 and 3 is outputted on the basis of the frame selection information. Therefore, for example, the partial image A written in the frame memory 2 and the partial image B written in the frame memory 3 are simultaneously displayed on the display unit I as window A and window B on the same screen, by which multi-window display can be realized.
The frame selection information stored in the frame selection control memory 4 is updated in the blanking period of the display units synchronization signal. When updating speed is low and updating does not end in the aforementioned blanking period, the image on the display screen of display unit 1 becomes out of order, so that the image is difficult to watch and/or decipher.
When an image is generated at a workstation or the like which is used for computer graphics, the output result is usually displayed on a display unit at the workstation. However, there is also a desire that the output result be displayed on a conventional TV set or recorded by a video tape recorder (VTR) for presentation or the like. In such case, it is necessary that the workstation output an image signal for TV viewing which has a different synchronization system such as, for example, an image signal of NTSC system, in addition to the display signal outputted to the display unit.
FIG. 2 shows a configuration of a conventional image signal output system. In the figure, a control section 11 controls the whole image signal output system. An input section 12 inputs image signals to a dual port video RAM 13. The input section consists of, for example, a signal processor.
The dual port video RAN (hereinafter abbreviated to VRAM) 13 is a large integrated circuit consisting of a RAM 14 for storing image signals for one screen and a serial access memory (hereinafter abbreviated to SAM) 15. The SAM 15, consisting of a shift register, reads image signals for one scanning line and outputs them as serial signals. A display section 16 displays the image signals outputted from the SAM 15 of the VRAM 13. A RAM port 17 is a terminal used when the image signals are written in the RAM 14. A SAM port 18 is a terminal used when the image signals are read from the SAM 15.
ADX denotes an address signal line through which an address signal for specifying the address of the RAM is sent from the control section 11 to the RAM 14. CTX denotes a control signal line through which a control signal for controlling the operation of the RAM 14 is sent from the control section 11 to the RAM 14. CL denotes a clock line through which a clock signal used for the shift operation of the SAM 15 is sent from the control section 11 to the SAM 15.
In the normal application mode of VRAM 13, the image signals for one screen are written in the RAM 14 through the RAM port 17, the image signals for one scanning line are transferred from the RAM 14 to the SAM 15, and the image signals transferred to the SAM 15 are read out through the SAM port 18.
To transfer the image signals for one scanning line from the RAM 14 to the SAM 15, the control section 11 outputs a row address which specifies a row in the RAM 14 in which the image signals for one scanning line to be transferred are stored to, the RAM 14 through the address signal line ADX. The control section 11 outputs the control signals for instructing the transfer to the RAM 14 via the control signal line CTX.
The period of time required for transferring the image signals for one scanning line from the RAM 14 to the SAM 15 is, for example, about 200 nanoseconds when the capacity of the RAM 14 is 1 megabit and the image signals for one scanning line constitute 1 kilobit.
The operation of the display signal output system shown in FIG. 2 now be described. First, the image signals for one screen are written in the RAM 14 from the input section 12 through the RAM port 17.
When the image signals for one screen are written in the RAM 14, the control section 11 transfers the image signals for one scanning line form the RAM 14 to the SAM 15. The period of time required for transferring from the RAM 14 to the SAM 15, being as short as about 200 nanoseconds, has no effect on the cycle of writing the image signals in the RAM 14.
The image signals for one scanning line transferred to the SAM 15 are sequentially sent to the display unit 16 from the SAM 15 through the SAM port 18 in accordance with the clock signal supplied from the control section 11 to the SAM 15, and the transferred image signals are displayed on the display section 16.
As described above, the image signals for the display section have so far been generated directly by the output from the SAM port 18 of the VRAM 13 which retains the image signals at the workstation.
However, there is a desire that the output result be displayed on a conventional TV set having a different display system or recorded in a VTR for presentation or the like, in addition to a desire that the output be displayed on a display unit (that is, the display section 16) at a workstation.
In this case, it is necessary that the VRAM 13 in the conventional image signal output system be read from different address at a different period and a plurality of image signals be sent.
However, the conventional system has a problem in that the image signals cannot be outputted simultaneously to a plurality of display units having a different scanning period since the SAM 15 of the VRAM 13 can retain only the image signals for one scanning line at a time.